The main aim of the present study was to know to what extent rumen protozoa could play a role in tryptophan (Trp) synthesis in the rumen microbial ecosystem. In 1990 Trp-synthsizing ability of protozoal suspension (P) was examined at first, then in 1991 that of bacterial suspension (B) for comparison, and in 1992 that in mixed suspension of protozoa and bacteria (BP) was examined to know a total quantity. Indolepyruvate (IPA) and indoleacetate (IAA) were used as substrates in these study and some metabolites besides Trp produced from these compounds were also examined. We will report here especially the results concerning Trp synthesis during a three-year research period comparing each other. After 12 h incubation of P, IPA (1mM) was converted to Trp by about 12% and Trp accumulated in the medium. Trp synthsized in P usually accumulates in the medium in this way, because protozoa cannot grow in this medium. We planned to demonstrate the synthesis of Trp using ^<14>C-IPA, but it was not produced in any company, and we gave it up. In P, there was no production of Trp from IAA. In B and BP, Trp was not accumulated so much, because these were growing systems. In these systems, Trp synthesis was examined using ^<14>C-IAA. After 12 h incubation, radioactive Trp was obviously detected in bacterial protein in B and the amount was estimated to be 5.7 nmol/ml. This means an occurrence of a reductive carboxylation of IAA to produce IPA and a consecutive tranamination of IPA to produce Trp in B. Radioacitve Trp was also detected in microbial protein in BP and the amount was 10.4 nmol/ml which was about 80% higher than that in B. Thus protozoa seemed to stimulate Trp synthesis in BP by means of transaminating ability characteristic to them. The velocity of reductive carboxylation of IAA by bacteria seemed to respond to that of transamination of IPA by both protozoa and bacteria.